1. The Field of the Invention
The present invention relates generally to x-ray tubes. More particularly, exemplary embodiments of the invention concern a shielded cathode assembly configured and arranged to control the unintended emission of x-rays from certain regions of an x-ray tube.
2. Related Technology
X-ray tubes are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. An x-ray tube typically includes a cathode assembly and an anode assembly disposed within an evacuated enclosure. The cathode assembly includes an electron source and the anode assembly includes a target surface that is oriented to receive electrons emitted by the electron source. During operation of the x-ray tube, an electric current is applied to the electron source, which causes electrons to be produced by thermionic emission. The electrons are then accelerated toward the target surface of the anode assembly by applying a high voltage potential between the cathode assembly and the anode assembly. When the electrons strike the anode assembly target surface, the kinetic energy of the electrons causes the production of x-rays. The x-rays ultimately exit the x-ray tube through a window in the x-ray tube, and interact with a material sample, patient, or other object.
Stationary anode x-ray tubes employ a stationary anode assembly that maintains the anode target surface stationary with respect to the stream of electrons produced by the cathode assembly electron source. In contrast, rotary anode x-ray tubes employ a rotary anode assembly that rotates portions of the anode's target surface into and out of the stream of electrons produced by the cathode assembly electron source. The target surfaces of both stationary and rotary anode x-ray tubes are generally angled, or otherwise oriented, so as to maximize the amount of x-rays produced at the target surface that can exit the x-ray tube via a window assembly.
Notwithstanding the orientation of both stationary and rotary anode target surfaces, x-rays nonetheless emanate in various directions from the target surface. Thus, while some x-rays do exit through the x-ray tube windows and are utilized as intended, some x-rays do not exit through the x-ray tube windows. X-rays that do not pass through the x-ray tube windows often penetrate instead into other areas of the x-ray tube, where the x-rays may, undesirably, be transmitted through other x-ray tube surfaces if sufficient measures to prevent the escape of those x-rays are not taken.
The escape of unusable x-rays from an x-ray tube is undesired as such x-rays can represent a significant source of x-ray contamination to x-ray tube surroundings. For instance, such unused x-rays can result in transmission of a relatively high level of radiation to x-ray tube operators.
In addition, unused x-rays can interfere with the imaging x-ray stream that is transmitted through the window. Such interference may compromise the quality of the images obtained with the x-ray device. For example, unused x-rays can impinge upon areas of the x-ray subject and interfere with the image being sought. The resulting interference may be manifested as clouding in the image.
While the problem can be realized throughout the tube environment, certain areas of the x-ray tube are especially susceptible to the impingement of non-window transmitted x-rays. For example, the area of the x-ray tube immediately behind the cathode assembly can be especially problematic. Since the electron source of the cathode assembly faces the target surface of the anode assembly, errant x-rays can emanate from the target surface toward the cathode assembly. Cathode assembly components are typically made of metals that are not effective at shielding x-rays, such as nickel or copper. X-rays typically pass through the cathode assembly without being absorbed, thus necessitating shielding materials behind the cathode assembly, either inside the x-ray tube or external to the x-ray tube.
Efforts to reduce the effects of unused x-rays have centered around the use of external shielding on x-ray tube structures. For instance, in many stationary anode x-ray tubes, a lining of lead shielding is placed about the inner surface of an outer housing, containing the x-ray tube, in order to absorb unused x-rays that are produced at the target surface and penetrate the evacuated enclosure of the x-ray tube.
The use of this type of shielding can be problematic however. For example, while such shielding can be effective at absorbing x-rays, lead is relatively heavy and substantially adds to the weight of the x-ray tube. This factor becomes important in applications where a relatively low x-ray tube weight is desired or even required.
Another problem relates to the tendency of x-rays to spread out somewhat as the x-rays travel further away from the target surface. In particular, because the lead lining is often placed relatively far away from the target surface of the anode, such as when the lining is attached to the outer housing located beyond the outer surface of the evacuated enclosure, relatively large amounts of lead must be used to cover significant portions of the enclosure surface in order to account for the spreading of the x-rays. In some cases, nearly the entire surface area of the evacuated enclosure must be covered by a lead lining to prevent x-ray emission from the x-ray tube. The addition of lead linings represents a significant cost in time and labor during x-ray tube manufacture.
In sum, there is an unmet need in the field of x-ray tubes to provide an x-ray tube structure that reduces the emission of errant x-rays, and that does so in a manner that minimizes the use of excessive, heavy internal or external shielding that significantly adds to the weight of the x-ray tube. Moreover, techniques for minimizing x-ray emissions in the region of the cathode assembly would be especially attractive.